1. Technical Field of the Invention
The invention relates to the reclamation and reuse of the abrasive slurries used in free-abrasive machining operations; and more particularly to the reclamation and reuse of abrasive slurries used with wire saws in the cutting of wafers from ingots of silicon and various other materials.
2. Background Art
The process of wire slicing for the production of wafers from hard crystal uses an abrasive "slurry" to accomplish the cutting operation in a wire saw. The slurry is a suspension of abrasive particles in a liquid called a "vehicle" or "carrier", which is applied to the wire during the slicing operation. The abrasive slurry causes channels to be ground in the crystal, separating the crystal into slices called wafers. The wafers produced by this method are used to make electronic devices, photovoltaic devices, optical windows, and other applications requiring that they have a particular thickness, flatness, and surface smoothness. The term "wire slicing" is partly a misnomer, since the wire does not do the slicing, but acts to transport the abrasive slurry, which slices by the process called "free-abrasive machining."
Free-abrasive machining is the general name for a process by which abrasive particles are suspended in a fluid medium used to transport them to the surface of a workpiece, typically of hard material like crystal or ceramic, where the particles abrade the workpiece in such a way as to create a feature in the surface of the workpiece or to separate the workpiece into two or more pieces. It is distinguished from bonded-abrasive machining, where the abrasive particles are bonded to a solid object, which is used to deliver the abrasive to the surface of the workpiece. Examples of free-abrasive machining processes include wire-sawing, ultrasonic machining, water-jet cutting, and sandblasting.
A wire saw is comprised of a collection of wires oriented under tension by a mechanical device that allows them to be driven in the same direction at high-speed. A wire saw drives hundreds of these wires simultaneously in a formation known as a "web", upon which the abrasive slurry is continuously deposited for transport to the workpiece. The slurry acts to abrade the workpiece, to flush the abraded particles away, and to cool the workpiece. The slurry is held in a sump and pumped onto the web. It is allowed to flow off of the web through a drain to return to the sump for recirculation to the web. A mechanical device slowly forces the workpiece, or "ingot" through the web, subdividing it. This method allows for the production of large numbers of uniformly sliced wafers.
During the slicing process the abrasive slurry becomes contaminated with ground material ("kerf") from the crystal or other material being sliced or sawn. In grinding practice, fine particles from machining are called "swarf". The particles of kerf are finer than the particles of abrasive that produce it.
In general, the ingot can be any material of any dimension, so long as it can be cut, ground, machined, or otherwise shaped by abrasive action. The kerf can come from any abrading process that takes place using a free-abrasive machining technique where the abrading particle is suspended in a fluid as a slurry, and the abrading process generates a particulate material that is finer than the abrading particle. The abrading process can be done by wire slicing, ultrasonic machining, lapping, polishing, water-jet cutting, or other means.
Abrasive is a major cost of operation for the production of wafers from hard materials by free-abrasive machining. The disposal of used abrasive is both an expense and an environmental issue for plants that use the process. The ability to recover and re-use abrasive that has been contaminated represents a potential cost-savings and reduction of waste volume of the abrasive materials.
The abrasive slurry used in wire saws is critical to the success of the wafer slicing operation. The quality of the abrasive and its liquid suspending medium or carrier is closely controlled. Special grades of abrasive and carrier are manufactured and sold, at premium prices, specifically to improve and stabilize slicing operations. Abrasive grades specifically produced for wafer slicing on a wire saw are controlled to a narrow size distribution. ESK F500.TM. abrasive material, for example, has a specification for particle size of 12.8+/-1.0 microns. The quality of the abrasive slurry used in the wire saw has come to be recognized as a key factor in its successful operation. The slurry components are metered carefully to insure consistency, and the flow-rate, density, viscosity, and temperature of the slurry are carefully monitored and controlled before and during the slicing process. Shin-Etsu Handotai Company's patent disclosure EP0798091A2, for example, describes methods to control slurry viscosity in a wire saw by dilution with water to produce uniform thickness in sliced wafers.
As the slurry gradually becomes contaminated with kerf through use, the thickness specifications of the wafers produced during the cutting process under comparable production conditions change. Specifically, wafer thickness, total thickness variation (TTV), and standard deviation (SD) of TTV change as abrasive slurry is used to slice successive batches of wafers. TTV is defined as the difference between the minimum and maximum thickness measured.
The general trend is that the thickness of the wafers increases during each successive batch, while TTV typically declines slightly during the second batch, and then increases considerably in the third and fourth consecutive batches of wafers. This is considered typical behavior for new abrasive, and it is commonly accepted in the industry that the second batch is typically the highest quality as measured by TTV. It has been suggested that a small amount of fine particle contamination is responsible for this decrease in TTV after the first batch, and has lead to a not uncommon practice of retaining a small amount of exhausted slurry to be added to fresh slurry as a pre-conditioning step to lower the TTV of the first batch.
The standard deviation of TTV typically increases after the first batch and then rapidly in the third and again in the fourth batch if a fourth is done. An increase in standard deviation indicates increasing variation in wafer quality within the batch of wafers produced during the batch, which indicates that the process is displaying less statistical control. These measurement techniques and trends are well-understood in the industry, and the wafer-quality phenomena as described here are generally accepted as facts.
Thickness is measured at five points on the wafer. Four measurements are taken a small distance from the edge of the wafer around its periphery, and one at the center. The average thickness is the average of the five measurements. The TTV is determined by the difference between the largest and smallest of the five measurements.
The amount of cutting that a given abrasive batch can do at acceptable quality can be extended by slowing the cutting process. This extends the time required for wafer production and thus raises the cost of production. Therefore, when wafer quality decreases below standards at an acceptable production rate, the slurry is disposed of as waste. For typical photovoltaic applications, for example, abrasive slurry is used three to four times in succession without modification and then discarded.
Inventions for the recovery of abrasives in grinding and blasting operations are described in previous patents. Grit-blasting operations in particular have patented processes for abrasive reclaim that include purification steps that provide a similar benefit to the abrasive by removing contamination from the abrasive. These inventions are typically pneumatic in nature, are not intended for wafer slicing, and can not be applied in the context of our invention.
A Varian Associates report, Slicing of Silicon into Sheet Material, Final Report, by J. R. Fleming et al, Sep. 21, 1979, studied wafer slicing with a gang saw (reciprocating blades) from 1976 to 1979 with cost reduction as the objective, and discussed abrasive lifetime and abrasive contamination during the slicing of wafers with a gang saw (reciprocating blades), and the possibility of recovery and purification of the abrasive in the slurry.
The authors attempted filtration and cycionic methods of abrasive recovery, but expressly declared they "did not work". They successfully recovered a portion of dry abrasive in a solid-bowl centrifuge. They also employed metal removal techniques and removed additional kerf material by solvent washing. The abrasive yield was 30% of the initial amount, although the authors predicted a higher percentage was achievable.
New abrasive was mixed with the recovered abrasive at a 2/1 ratio and used to prepare a new batch of slurry which was used successfully for another test cutting of wafers. The report concludes that multiple recoveries and recombining of used abrasive with new abrasive at the same ratio for use in slurries should have no detrimental effects on cutting time or wafer thickness, while achieving desired cost reductions.
The emphasis of this work was on cost reduction through the recovery and re combination of such amount of the used abrasive material as will not negatively impact production time or wafer thickness. Other approaches to the free-abrasive machining process and the recycling of used slurry, with other potential benefits, have apparently been obscured by the tight focus on cost reduction through recovery of abrasive material of this and similar efforts.